1. Field of the Invention
This invention relates to recovery of hydrogen-rich gas streams from hydrogen-containing gas streams and particularly relates to selective recovery of at least 95% purity hydrogen from a wide variety of refinery off-gases by treatment in accordance with the Mehra Process.
2. Review of the Prior Art
As crudes having higher sulfur content and higher carbon-to-hydrogen ratio continue to be processed and as stricter environmental regulations requiring lower sulfur content are passed, the hydrogen demand is expected to grow. Even though a substantial portion of this increased demand will be met by steam reforming of light hydrocarbons and partial oxidation of heavy hydrocarbons, upgrading of existing off-gas streams is a viable alternative.
There are many small to medium size off-gas streams that contain hydrogen and heavier hydrocarbons which are currently being sent to the fuel systems of petroleum refineries. A summary of various hydrogen source streams containing approximate concentrations of hydrogen is shown in Table I. In most of the refinery and petrochemical applications where hydrogen is used as a reactant, the desired makeup hydrogen has a purity of about 95%. In order to prevent the build-up of reaction byproducts, such as methane, a portion of the recycle stream is customarily purged. Even though such a stream is relatively small, its concentration of hydrogen represents a loss which must be offset by additional hydrogen makeup.
TABLE I ______________________________________ Sources of Hydrogen Off-Gas Streams Approximate Hydrogen Industry Source Concentration ______________________________________ Refining HT Purge 25-35 FCC Gas 10-15 Cascade Reject 50-60 Methanol Purge Gas 70-80 Ethylene By-product H.sub.2 60-90 Cracked Gas 10-35 Coke Oven Product Gas 0-5 LPG Dehydrogenation Product Gas 58 Toluene HDA H.sub.2 Purge 57 Cyclohexane H.sub.2 Purge 42 Carbon Black Product Gas 7 Formaldehyde By-product H.sub.2 18 Ammonia Purge Gas 60 ______________________________________
Several processes have been used and are currently available for upgrading the quality of such off-gas streams. These processes include cryogenic separation, catalytic purification, pressure swing adsorption, membrane separation, and absorption with a selective solvent. Selection of a suitable process depends upon many factors, some of which are the hydrogen product purity that is desired, hydrogen recovery levels, available pressure drop, pretreatment requirements, off-gas composition, impact of reaction products remaining in the hydrogen product, and turn down capability of the selected process.
The bulk of the industrial hydrogen manufactured in the United States uses the process of steam reforming of natural gas according to the equation CH.sub.4 +2H.sub.2 O.fwdarw.CO.sub.2 +4H.sub.2. Other processes utilize partial oxidation of resids, coal gasification, and water hydrolysis, but when proceeding from natural gas to liquid hydrocarbons and then to solid feed stocks, the processing difficulties and manufacturing costs increase.
The impurities usually found in raw hydrogen are C0.sub.2, CO, 0.sub.2, N.sub.2, H.sub.2 0, CH.sub.4, H.sub.2 S, and higher hydrocarbons. These impurities can be removed by shift catalysis, H.sub.2 S and C0.sub.2 removal, PSA process, and nitrogen wash. Upgrading of various refinery waste gases is nearly always more economical than hydrogen production by steam reforming. Composition of the raw gas and the amount of impurities that can be tolerated in the product generally determine the selection of the most suitable process for purification.
U.S. Pat. No. 1,875,311 teaches an absorption process for recovering olefins from still gases formed by distillation, cracking, and the like in oil refinery operations and containing an average of about 35% olefins and about 65% saturated hydrocarbons and hydrogen. This process utilizes water-soluble absorbents, such as 95% ethyl alcohol, glycerine, and glycol, as its absorbent medium, based upon ethylene, for example, being about 5.5 to 6 times more soluble in 95% ethyl alcohol than the saturated hydrocarbons.
An absorption process is disclosed in U.S. Pat. No. 3,213,151 for recovering a recycle stream of 50% hydrogen from a gaseous mixture, comprising hydrogen, methane, and normally liquid hydrocarbons, by absorption with pentanes.
A process is disclosed in U.S. Pat. No. 3,291,849 in which toluene, mixed with other alkyl benzenes, is produced as a lean oil which is used in an absorber to purify a make-up hydrogen stream from a catalytic reformer.
A process is disclosed in U.S. Pat. No. 3,349,145 which comprises treatment of an off-gas stream from a catalytic naphtha reformer which contains 50-90 mole % hydrogen, with the remainder being essentially C.sub.1 -C.sub.6 paraffins. The off-gas stream is passed countercurrently to an absorber oil in an absorber-stripper system. The absorber oil consists essentially of a mixture of C.sub.9 + aromatic hydrocarbons (trimethylbenzenes, propylbenzenes, cumene, naphthalene, and diphenyl) having an average molecular weight of 125, a gravity of 20.degree. API, and a hydrogen equivalency of 1.5. A reformer off-gas stream of 75 mol % hydrogen purity is exemplarily upgraded to 82.6 mol % hydrogen purity.
Acetylene is described in U.S. Pat. No. 3,686,344 as being washed out of a cracked gas containing 56% hydrogen and 8% acetylene with a physical solvent such as dimethyl formamide, butyro lactone, tetraethyl glycol dimethyl ether, and preferably, N-methyl pyrrolidone.
A process is described in U.S. Pat. No. 3,877,893 for treating a combustible synthesis gas mixture, derived from fossil fuels by carbonization, cracking, partial combustion, or water gas reaction and containing acid gas and hydrogen, by passing a dialkyl ether of a polyethylene glycol solvent in intimate contact therewith through an absorption zone. The gas mixture exemplarily contains 43 mol % hydrogen.
U.S. Pat. No. 4,552,572 relates to purification of raw gases derived from coal by high temperature gasification. Suitable purification solvents must have preferential selectivity for hydrogen sulfide over carbon dioxide. They include methanol, N-methyl pyrrolidone, and dimethyl ether of polyethylene glycol. Commonly, the raw gas intended for synthesis is divided into two parts, one of which is passed through a shift reactor to convert a major portion of its carbon monoxide to hydrogen by the shift reaction: CO +H.sub.2 0.fwdarw.C0.sub.2 +H.sub.2. As the purification treatments remove impurities, including C0.sub.2, the shifted gas, which is rich in hydrogen, and the unshifted gas, which is rich in carbon monoxide, may be blended to produce the ratio of hydrogen to carbon monoxide required for a specific synthesis.
Purification of gases may be necessary in addition to blending, however, depending on the ultimate use thereof. For example, for fuel use, an unshifted gas is used in which 90% or more of the sulfur compounds is removed.
For methanol synthesis, shifted and unshifted gases are blended to obtain a ratio of H.sub.2 to CO of 2, and the blend is purified by reducing the CO content to about 5 mole % in the feed gas to methanol synthesis.
For hydrogen use, the gas is shifted to the maximum practical extent. Desulfurization is carried to 1 ppm or less, and CO.sub.2 is carried to less than 1 mole % before methanation.
For ammonia production, the purification requirements are similar, but a liquid nitrogen wash may be used instead of methanation.
If natural gas is the desired product, the H.sub.2 -to-CO ratio needed is 3, according to the equation, CO+3H.sub.2 .fwdarw.CH.sub.4 +H.sub.2 0. However, for methane synthesis, an alternate reaction is 4CO+2H.sub.2 O.fwdarw.CH.sub.4 +3CO.sub.2. By properly combining these reactions in the presence of a suitable catalyst, methane can be synthesized without a separate shift step.
A new selective solvent process has recently been available for the extraction of hydrocarbon liquids from natural gas streams. This process, known as the Mehra Process, has also been available for processing inert-rich hydrocarbon gases.
The Mehra Process utilizes a preferential physical solvent for the removal and recovery of desirable hydrocarbons from a gas stream. In the presence of a selected preferential physical solvent, the relative volatility behavior of hydrocarbons is enhanced. The selected solvent also has high loading capacity for desirable hydrocarbons. Since the Mehra Process combines solvent selectivity with its hydrocarbon solubility, the selective hydrocarbon removal step in this process is EXTRACTION.
If hydrocarbons heavier than methane, such as ethane, ethylene, propane, propylene, butanes, etc., are present, they can be selectively removed from the gas stream as a combined liquids product. The hydrocarbon component recoveries can be adjusted to any degree varying in the range of 2-98+% for methane, 2-90+% for ethane, and 2-100% for propane and heavier hydrocarbons.
In the Mehra Process, methane is generally considered to be one of the undesirable hydrocarbons which leaves the process as residual gas. However, as taught in U.S. Pat. No. 4,526,594, the residue gas can be selectively purified to become the product gas. The Mehra Process accordingly provides flexible recovery to a selected degree of only economically desirable hydrocarbons as a hydrocarbon liquids product or as a product gas.
A wide variety of gaseous streams are to be found in petroleum refineries. Some streams are integral parts of a specific process, e.g., they are recycled from a fractionating column to a reactor. Such a recycle stream may be an impure hydrogen stream which must be purified before returning to the reactor and/or combining with a make-up hydrogen stream. Other such gaseous streams may be a byproduct of a major refinery process and may be sent to one or more other processes which are nearby and require a hydrogen feed stream. Purification of these byproduct streams is generally also needed. For example, the byproduct hydrogen stream from an ethylene cracking plant may have a hydrogen content of 75 mol % and may be initially needed as feed to a hydrodealkylation process requiring 95 mol % hydrogen. Or a change in process conditions at a nearby hydroforming plant may create a demand for 99 mol % hydrogen and consequent purification of a 90% hydrogen byproduct stream, for example, that happens to be available.
There is clearly a need in such circumstances to be able to change selectively from one hydrogen purity to another without having to change equipment specifications.